Gas turbine engines have been well known in the art for many years, and are engines in which a shaft containing a row of compressor blades serves as the drive shaft for generating a thrust output from the engine. Such engines are typically employed on aircraft, and can either be used in combination with propeller drive system to form a turboprop system, or without a propeller, as in a turboshaft, turbojet or turbofan system. Engines incorporating compressor blades on a single drive shaft are known as single axial flow compressor engines. Another type of engine is one which may include two coaxial drive shafts, in what is referred to as a dual axial flow compressor engine. In such an engine, rows of low pressure compressor blades are connected by a first drive shaft to a drive turbine. Downstream of these rows of low pressure compressor blades are rows of high pressure compressor blades connected to a second coaxial drive shaft which is driven by separate drive turbines.
Whether the engine is the single axial or dual axial type, the drive shafts must be capable of rotating at tens of thousands of Rpm's for hours at a time, under intense variations in temperature, acceleration, centrifugal stress, axial stress, etc.
After years of shaft usage, circumstances have arisen where one of the drive shafts separates from the remaining portion of the shaft. Because the drive shaft is rotating at such high rate of speed, failure or "decoupling" of the shaft will occur suddenly and rapidly. When a gas turbine engine experiences a shaft failure, the entire failure sequence may occur in less than one second, and produce a sudden catastrophic failure of the engine in which the rotating components of the engine upstream of the failure will suddenly decelerate, while rotating components downstream of the failure will begin to accelerate uncontrollably. The uncontrolled acceleration downstream of the failure poses the greatest hazard, because the rotational velocity of these components may reach a point where the centrifugal forces on these components cause them to shear away from the drive shaft, and impact the engine housing risking possible non-containment of these components within the engine housing. On a jet aircraft, such a non-containment could result in serious damage to the remaining portions of the engine, as well as damage to the aircraft fuselage.
Various attempts have been made to contain a component burst through the engine housing. In one such attempt, a solid containment ring formed of high strength material, such as a nickel cobalt alloy has been integrated into the outer engine housing to circumferentially surround the rotating components of the engine. Although such containment rings have been successful in containing fragmented components within the engine housing, they add a significant amount of additional weight to the engine, thus sacrificing fuel economy and passenger capacity. The trapping of failed engine components within the engine itself also results extensive, irreparable damage to the engine, often requiring that the entire engine be replaced after such a failure, thus adding substantial cost to the operation of the aircraft.
A need therefore exists to develop a warning protocol to identify the immediate signs of a drive shaft failure, and shut down the engine before the portions of the drive shaft downstream of the failure accelerate to a level that will place excessive stresses on the rotating components. Because these warning signs will appear only fractions of a second before the engine components start to fragment, it is evident that such a warning protocol must also be automated, preferably in the form of a control logic utilized by a high speed on board processor. If it becomes possible to shut down the engine while it is displaying the early warning signs of shaft failure, and before any component fragmentation occurs, the need for using heavy containment rings can be eliminated. In addition, damage to the engine resulting from the high speed component fragmentation can be eliminated as well. Most importantly, however, the safety of the operational engine can be significantly improved, since the chances of component fragmentation can be eliminated, thus improving the safety and integrity of the passenger compartment.